Anorthoclase megacrysts: physical conditions of formation

Abstract

SUMMARY. Previous interpretations relating to precipitation of anorthoclase megacrysts from alkaline basaltic magmas under high-pressure conditions are contradicted, It is proposed that these feldspars were crystallized above the solvus from hydrous melts at shallow depths, under low-pressure equilibrium conditions. These melts had been considerably enriched with volatiles of sodium, strontium, and barium, and to some extent also of rubidium.

Ba partitioning and the origin of anorthoclase megacrysts in basalitic rocks

“…However, more recent isotopic data (O'Reilly and Griffin, 1990) clearly demonstrate that within a given volcanic province, although individual anorthoclase/host pairs exhibit isotopic disequilibrium, the variations in the (87Sr/S6Sr)i ratio for anorthoclase are within the range of (878r/86Sr)i ratios of basaltic rocks in that province. Aspen et al (1990) Kutolin andFrolova (1970), Irving (1971), Bahat (1979), Mason et al (1982, Irving and Frey (1984) and this work. Data sources for Ba in rocks: Knutson (1975), Baker et al (1977, Rafferty and Heming (1979), White et al (1979), Ghent et al (1980, Wass (1980), Day (1983), Briggs and Goles (1984), Irving and Frey (1984), Nelson and Carmichael (1984), McDonough et al (1985) and Nelson et al (1987).…”

“…Considering the effect of feldspar composition on Ba partitioning, the natural anorthoclases must have lower partition coefficients for Ba since DBa decreases with increasing Na/K ratio of alkali feldspar (Guo and Green, 1989). Direct extrapo- Binns et al (1970), Kutoliu and Frolova (1970), Hoffer and Hoffer (1973), Irving (1971Irving ( , 1974, Chapman (1976), Bahat (1979), Mason et al (1982), Irving and Frey (1985), and this work.…”

“…Previous interpretations of the origin of anorthoclase megacrysts fall into two categories: either (1) anorthoclase together with the associated pyroxene and amphibole megacrysts crystallised from basaltic magmas as phenocrysts at high pressure and were transported with the same magma during eruption; according to this hypothesis, the present host basaltic rocks represent the parent magmas for the anorthoclase megacrysts (Binns et al, 1970;Kutolin and Frolova, 1970;Hoffer and Hoffer, 1973;Laughlin et al, 1971); or (2) unlike the associated pyroxene and amphibole megacrysts, anorthoclase crystallised from a more evolved liquid at relatively lower pressure and was then picked up and carried to the surface by a later surge of alkalic basaltic magma in the overall eruptive episode. Thus the present host basaltic rocks do not represent the parent magmas of the anorthoclases (Irving, 1974;Chapman and Powell, 1976;Bahat, 1979;Upton et al, 1983;Irving and Frey, 1984;Aspen et al, 1990). The nature of this 'evolved melt' is speculative.…”

The origins of contrasting zoning patterns in hyalophane from olivine leucitites, Northeast China

“…Other data sources: Lucchini et al (1969); Hoffer and Hoffer (1973);Irving (1974); Laughlin et al (1974); Bahat (1979); Mason et al (1982); Irving and Frey (1984); Aspen et al (1990). For the sake of clarity, 50% data points for megacrysts from Australia, West USA and Scotland are not plotted.…”

“…Based on petrographic evidence, phase equilibrium experiments, and trace element and radiogenic isotopic data, it has been generally accepted that alkali feldspar megacrysts are not high-pressure phenocrysts of their hosts (e.g. Stuckless and Irving, 1976;Bahat, 1979;Irving and Frey, 1984). Instead, it is thought that they crystallized from benmoreitic (Guo et al, 1992) or phonolitic (Aspen et al, 1990) magmas at moderate or high pressures and subsequently were entrained by more primitive basaltic hosts during ascent at high temperatures.…”